Applicators (for example, fountain pens) including an ink tank for accommodating ink have been known in the art. Various problems occur when a pressure fluctuation is generated in the ink tank due to variations in the environmental conditions (such as pressure or ambient temperature) under which such an applicator is used. For example, if the pressure in the ink tank is increased, the ink leaks from a nib. If the pressure in the ink tank is decreased, on the other hand, the ink has a difficulty in being discharged from the nib.
To solve such a problem, a writing instrument having a pressure fluctuation buffering mechanism including: a comb groove for retaining ink; an ink flow groove provided perpendicular to the comb groove; and an air flow groove provided perpendicular to the comb groove to solve the problem caused by pressure fluctuation in an ink tank has been known in the art (for example, Japanese Examined Utility Model Application Publication No. Sho. 62-028458).